Manufacturing galvannealed sheets by heat-treating electrolytically finished sheets

ABSTRACT

The invention relates to a method for manufacturing a steel strip with a cathodic corrosion protection coating, in which the steel strip is hot rolled and then cold rolled; the steel strip is subjected to an electrolytic galvanization and after the electrolytic galvanization, the steel strip is heat treated in a bell-type annealing furnace at temperatures of 250° C. to 350° C. for a period of 4 to 48 hours and this produces a zinc/iron layer.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing coated steel sheets.

BACKGROUND OF THE INVENTION

It is known to produce steel sheets with a coating composed of a metal to protect the steel from corrosion.

These are basically divided into so-called barrier protection coatings and cathodically acting protection coatings.

Barrier protection coatings are in particular protective coatings that are composed of aluminum, tin, or chromium; to produce them, for example an aluminum alloy is deposited onto a steel strip by means of a so-called hot-dip coating process.

The most widely used cathodically acting corrosion protection layer is a zinc coating; in particular, its action is based on the fact that when the zinc layer on the steel is damaged all the way down to the steel substrate, the zinc, as the chemically less noble metal, is corroded first, thus protecting the steel substrate.

Zinc coatings include pure zinc coatings, zinc coatings with a low aluminum content, zinc coatings with an aluminum content of about 5% (Galfan), and zinc aluminum coatings with approximately half zinc and half aluminum.

These coatings are likewise applied using the hot dipping method in which a preheated steel strip is conveyed through a bath of zinc or zinc alloy.

So-called galvannealed layers represent a special case in which first, a hot-dip galvanization is used to deposit a zinc layer or zinc alloy layer onto a steel substrate and then an annealing step is carried out so that a diffusion reaction occurs between the zinc on the one hand and the iron of the steel substrate on the other so that a zinc/iron alloy layer forms. Such a layer is referred to as the galvannealed layer.

Such a galvannealed layer is produced at annealing temperatures of 480° C. to 600° C. in continuous annealing furnaces through which the strip passes after the galvanization.

DE 10 2007 031 91 96 A1 relates to a method for producing flexibly rolled strip stock with a cathodic corrosion protection layer; in this method, flexibly rolled material, i.e. steel strip with different steel strip thicknesses over its length, is among other things electrolytically galvanized and then annealed, with the annealing treatment being performed at <420° C.

DE 10 2007 013 739 has disclosed a method for flexibly rolling coated steel strips; these steel strips can also be electrolytically galvanized and continuously annealed.

DE 10 2004 023 886 B4 has disclosed a method and an apparatus for finishing flexibly rolled strip stock in which flexibly rolled strip stock with a periodically varying material thickness is conveyed continuously through a processing line composed of an annealing section, a quenching unit, a preheating unit, and a zinc pot, and is thus heat treated and hot-dip galvanized; the inlet temperature of the flexibly rolled strip into the zinc pot is regulated to a constant value by varying the heat energy in the preheating unit as a function of the strip thickness and the zinc depositing thickness is regulated to a constant value by varying the distance of outlet nozzles from the flexibly rolled strip as a function of the strip thickness.

DE 10 2004 023 886 B4 has disclosed a method and an apparatus for finishing flexibly rolled strip stock in which the cold-rolled strip is to have a property profile that is matched to its thickness progression, where a first annealing treatment is carried out at a temperature between 500° C. and 600° C., after which the cold-rolled strip is rolled to a predefined thickness progression so that in the rolling direction, the flexible cold-rolled strip has at least one region of greater thickness and one region of lesser thickness; this is then followed by a second annealing treatment in which the temperature is higher than in the first annealing treatment.

It is, however, problematic to apply a so-called galvannealed layer onto strips that have been flexibly rolled in this way. The application of the galvannealed layer is usually carried out by means of a hot-dip galvanization followed by a continuous inline heat treatment. But this method cannot be used for providing a galvannealed coating to steel grades that are impossible or very expensive to manufacture by means of a hot-dip galvanization. These include isotropic steels, higher strength steels with strengths >1,000 MPa, and the aforementioned flexibly rolled strips. Specifically with flexibly rolled strips in a hot-dip galvanizing system, the different sheet thicknesses result in both irregular mechanical properties and differing zinc/iron growth rates since in inline processes, the thick regions naturally heat up differently than the thin regions, necessarily resulting in correspondingly different diffusion speeds between zinc and iron.

There are other reasons, however, why manufacturing a flexibly rolled strip with a galvannealed coating in a hot-dip galvanizing system seems hardly feasible. Even with an induction galvannealing system, the required accuracy of the output control (variation of the coil power by a factor of up to two and more within a few centimeters of positioning accuracy) would be difficult to achieve.

The object of the invention is to create a method for manufacturing flexibly rolled or isotropic or higher strength galvannealing-coated steel strips.

SUMMARY OF THE INVENTION

According to the invention, a method for manufacturing a steel strip with a cathodic corrosion protection coating includes hot rolling the steel strip and subsequently cold rolling the steel strip; subjecting the steel strip to an electrolytic galvanization; and, after the electrolytic galvanization, heat treating the steel strip in a bell-type annealing furnace at temperatures of 250° C. to 350° C. for a period of 4 to 48 hours to produce a zinc/iron layer.

The method may also include flexibly cold rolling the steel strip in such a way that the steel strip has periodically different thicknesses and/or widths over its length and/or a steel strip with isotropic or higher-strength properties is used.

In certain embodiments, before the electrolytic galvanization, the flexibly rolled strip may be subjected to a recrystallization annealing at 550° C. to 650° C. in the bell-type annealing furnace or in a continuous annealing furnace.

A layer thickness of the electrolytically deposited zinc coating may be between 2 μm and 10 μm.

The method may also include annealing the steel strip in order to convert the zinc layer into a zinc/iron layer in such a way that the zinc/iron layer contains a maximum of 30% iron.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, the problems arising due to different temperatures in the flexibly rolled strip are circumvented in that the strip is not hot-dip galvanized but instead electrolytically galvanized, thus avoiding different build-up heights of the zinc and iron phases due to temperature differences in the strip during the hot-dip galvanization. The same approach can be used to avoid problems when hot-dip galvanizing isotropic or higher strength steels.

Also according to the invention, the galvannealed coating is produced not continuously in an inline fashion, but instead, the galvannealed layer formation takes place under protective gas in a bell-type annealing furnace. Also according to the invention, the temperature of the annealing is reduced; according to the invention, temperatures of about 300° C. are maintained for holding times of approximately 20 hours.

The slow heating in the bell-type annealing furnace results in a uniform heating of the strip so that the zinc iron-reaction also occurs very uniformly in strip regions with different thicknesses.

According to the invention, a strip is flexibly cold rolled, is subjected to a recrystallization annealing in a bell-type annealing furnace at approximately 650° C. for 24 hours, following which the strip is then temper rolled and electrolytically galvanized and then undergoes a galvannealing step in the bell-type annealing furnace.

The invention therefore has the advantage that a steel strip with periodically varying sheet thickness (flexibly rolled strip (tailor rolled blank)) can be provided with a high-quality cathodic corrosion protection, which has good weldability; advantageously, both isotropic and higher strength steels as well as other steel grades can likewise be provided with a galvannealed layer.

It is also advantageous that steel sheets that are galvannealed according to the invention can be produced with very thin galvannealed coatings, which is made possible on the one hand by the electrolytic galvanization and on the other hand, by the gentle cooling.

According to the invention, isotropic steels and higher strength steels that can only be hot-dip galvanized with difficulty can be provided with a galvanized coating if the electrolytic galvanization is followed by the above-described bell-type annealing furnace step according to the invention. 

1. A method for manufacturing a steel strip with a cathodic corrosion protection coating, comprising: hot rolling the steel strip and subsequently cold rolling the steel strip; subjecting the steel strip to an electrolytic galvanization; and after the electrolytic galvanization, heat treating the steel strip in a bell-type annealing furnace at temperatures of 250° C. to 350° C. for a period of 4 to 48 hours to produce a zinc/iron layer.
 2. The method as recited in claim 1, comprising flexibly cold rolling the steel strip in such a way that the steel strip has periodically different thicknesses and/or widths over its length and/or a steel strip with isotropic or higher-strength properties is used.
 3. The method as recited in claim 1, wherein before the electrolytic galvanization, the flexibly rolled strip is subjected to a recrystallization annealing at 550° C. to 650° C. in the bell-type annealing furnace or in a continuous annealing furnace.
 4. The method as recited in claim 1, wherein a layer thickness of the electrolytically deposited zinc coating is between 2 μm and 10 μm.
 5. The method as recited in claim 4, comprising annealing the steel strip in order to convert the zinc layer into a zinc/iron layer in such a way that the zinc/iron layer contains a maximum of 30% iron. 